You are in Building 27 on the third floor. It is time to calibrate a pressure transmitter. You have just finished the highest pressure point of the test. Now, it’s time to bring it back down. The easiest way to reduce the pressure is to vent the pressure pump. So, when you vent the pump, there is process fluid everywhere. That is not what you expected. How did this happen?
Calibrating pressure instruments has its own unique challenges. The illustration above depicts a common occurrence during pressure instrument calibration. When process fluid erupts from the instrument, it can be a big mess to clean up. To avoid this literal mess and improper calibrations, there some sensible tips and principles to take into account. Here are three of the most common pitfalls that occur during pressure calibrations:
- One such challenge is knowing where to vent after you have pressured up your test line. In the scenario described above, a three-legged hose is used with one end going to the device under test (DUT), one end to the pressure source (a hand pump, in this case) and one end to the pressure port on the calibrator. Venting a line under pressure creates a low pressure area similar to a vacuum effect on the contents of the line. It could be atmospheric air, nitrogen from a bottle or a process fluid (if present in the line, capillary or bourdon tube). If the process fluid is corrosive or dangerous, this can be an especially bad situation. Now you have to clean out the pump and probably the pressure transducer. However, you can avoid this in the future by venting at the DUT.
- Another common challenge is finding zero. Isn’t zero always a mathematical zero? No. Not when you are dealing with gauge pressure. Absolute pressure has a zero, which would be a perfect vacuum found in space. When you zero a gauge pressure instrument, you zero it to the atmospheric pressure at that moment. Because this barometric pressure changes, your zero changes. This should be accounted for as a slight deviation due to random error in the process tolerance.
- In looking further at pressure testing, the orientation of your DUT plays an important role during calibration and installation. A common design of pressure transmitters is to use a protective diaphragm seal with silicone fluid on the other side to register the pressure applied to the actual instrument sensor. Gravity will have an effect on the measurement with this arrangement. To demonstrate this effect, calibrate a pressure transmitter lying flat in a horizontal position. Then, simply change the position of the DUT to upright or vertical and run the test points again. You will notice a dramatic effect.
Although there are other factors to consider, identifying where to vent the line, understanding the type of pressure being measured and the orientation of the DUT will help you sidestep some of the most common challenges in pressure instrument calibration. When you vent the line at the DUT, catastrophic messes are circumvented. Measurements are the most accurate when you understand the type of pressure you are working with. Calibrating the instrument in the same position as it is installed will help ensure that calibration is completed correctly. Overall, avoiding these common mistakes will save you time, as well as improve quality and accuracy of calibrations.